As radio spectrum is at a premium, spectrally efficient transmission techniques are required in order to provide users with as many multimedia broadcast/multicast services as possible, thereby providing mobile phone users (often referred to as subscribers) with the widest choice of services. It is known that multimedia broadcast/multicast services may be carried over cellular networks, in a similar manner to conventional terrestrial television/radio transmissions.
In order to provide enhanced communication services, the 3rd generation cellular communication systems are designed to support a variety of different and enhanced services. One such enhanced service is multimedia broadcast and multicast services (MBMS). The demand for multimedia services that can be received via mobile phones and other handheld devices is set to grow rapidly over the next, few years. Multimedia services, due to the nature of the data content that is to be communicated, require a high bandwidth.
Technologies for delivering multimedia services over cellular systems, such as the Mobile Broadcast and Multicast Service (MBMS) for UMTS, have been developed over the past few years. Examples of MBMS services and applications include multimedia broadcast, i.e. mobile TV, audio, video, etc.
In order to achieve efficient transmission, two delivery modes have been defined for MBMS delivery in 3GPP mobile communication system. The typical and most cost-effective approach in the provision of multimedia services is to ‘broadcast’ the multimedia signals, as opposed to send the multimedia signals in an uni-cast (i.e. point-to-point) manner, where a dedicated radio bearer to a particular UE is provided. MBMS technology is designed to transmit data traffic from a content server (often referred to as a data source) to multiple destination user terminals (UEs) in a cellular/mobile communication system. Thus, typically, tens of channels carrying say, news, movies, sports, etc. may be broadcast simultaneously over a communication network.
The decision of the delivery mode is made at a network controller based on the number of users that have activated the service in the coverage area of the network controller. If the number of UEs that have activated the service is larger than a pre-set threshold value, p-t-m transmission is selected and used. Otherwise, the service is delivered over the p-t-p radio bearers. This is done in order to optimise the efficiency of delivering the MBMS data content according to the number of participating users. In some instances, the selection may be made on the potential interest in a particular service, i.e. the network controller asks served UEs whether they are interested in a particular MBMS service, and dependent upon the response, a (p-t-p or p-t-m) transmission delivery mode is selected. Thus, if the number of users interested in a service is less than a pre-set threshold value, then p-t-p mode is used as this is more radio efficient. However, the pre-set threshold value is mainly a function of the radio technology employed, and in most practical deployment scenarios is typically arranged to be less than two or three users in a cell.
The selection of a delivery mode for a MBMS service in a coverage area is termed “counting” procedure. The network controller initiates a counting procedure by sending a counting request message in a downlink (DL) channel over the Main Control Channel (MCCH). Notably, Layer-3 signally is used for the communication between the network controller and the UE during the counting procedure. Once a UE detects that the counting procedure is on-going for the specific MBMS service that the UE has activated, the UE replies to the counting request by sending a counting response message over the random access channel (RACH) to the network controller. As there may be many users who have activated (or are interested in) a particular service in a cell, the number of simultaneous counting replies over a RACH may overload the RACH resources. This also impacts the RACH access by other (non-MBMS) users, which is problematic in the efficient provision of resources.
The RACH is a contention-based access channel, where the physical channel resources to be used for RACH is defined by the network and broadcast in the cell together with System Information. Thus, the RACH channel information is known to a UE in a system. When the UE needs to communicate to the network, whilst the UE has not been assigned dedicated resources to be used by the network, the UE uses a RACH channel. Hence, the UE access on RACH is random and may result in UE collisions/contention on RACH channel.
To avoid the congestion on the RACH due to a large number of counting responses, the network controller may perform access control during the counting procedure in the UMTS Terrestrial Radio Access Network (UTRAN). This uses an access prioritisation mechanism with use of an access probability factor. If the access probability factor is large, the UE has a higher chance of RACH access for submitting a counting response. Thus, the access probability factor is transmitted to the UE, together with the counting request message over MCCH. All UEs read the MCCH. Two different access probability factors are defined as:
(i) access_probability_factor_idle (as used by the idle state UEs); and
(ii) access_probability_factor_connected (as used by the connected state UEs (i.e. those active on URA_PCH, CELL_PCH or CELL_FACH state).
The counting response message also takes two forms, dependent upon the UE state (i.e. RRC_connected or RRC_Idle). The RRC_Idle state UEs have no connection to the network and reply to the counting request by establishing a radio resource connection (RRC) with the establishment cause (indication) set to ‘MBMS reception’. The RRC_Connected UEs have connection to the network and are therefore configured to send a cell update message with the establishment cause (indication) also set to MBMS reception.
Thus, depending on their state, both connected and Idle UEs use either access_probability_factor_idle or access_probability_factor_connected for RACH access. Normally, access_probability_factor_connected provides a higher probability of access (due to the connected state UEs already being connected to the network and therefore able to send a quicker response to the network) than access_probability_factor_idle.
Referring now to FIG. 1, the known mechanism for allocating MBMS resources to a UE 150 from a Radio Network Controller (RNC) 102 via a Node-B in UMTS is illustrated. The RNC 102 and Node-B are illustrated as comprising a number of logic elements that can handle signalling at various levels of the well known OSI model, for example: Layer-1 air-interface signalling 112, Layer-2 medium access control signalling 110, Layer-2 resource link control 108 and Layer-3 signalling 106. Similarly, the UE 150 comprises logic elements that can handle signalling at various levels of the well known OSI model at the subscriber end of the communication link, for example: Layer-1 air-interface signalling 152, Layer-2 medium access control signalling 154, Layer-2 resource link control 156 and Layer-3 signalling 158. The various layers used for signalling communication are self explanatory from FIG. 1, and will thus not be described further here.
The known process starts with the radio resource management (RRM) 104 making a decision to commence a counting operation 100 for a particular MBMS service by sending an instruction 116 to Layer-3 signalling. A RRC-counting request (RRC-CR) message 118 is followed by a RRC-CR message on the MCCH 120 and in turn by an RRC-CR message on the MBMS transport channel (MCH) 122.
An RRC-CR message 124 is then transmit across the air-interface to the UE 150, where it is received and processed. The air-interface (Layer-1) signalling message is converted into a RRC-CR message 160 on the MCH, and in turn an RRC-CR message 162 on the MCCH (MBMS control channel). Following receipt of the Layer-3 RRC-CR message 164 in the UE, a decision 166 on whether the UE is interested in the counting procedure can be made.
In response, the UE initiates a Layer-3 RRC-connection request (RRC-CoR) message 168, in turn followed by Layer-2 RRC-CoR messages 170, 172 requesting a RACH access 175. An RRC-CoR message 176 is then sent to Layer-1 signalling on the RACH and an air-interface RRC-CoR message 126 sent to the network controller 102. The network controller 102 receives the air-interface RRC-CoR message 126 and processes this via Layer-2 RRC-CoR message 128 on the CCCH and Layer-2 RRC-CoR message 130, with an admission control message 132 to admission control logic 134 in the RRM 104.
The admission control 134 in the RRM 104 sends an instruction 136 to Layer-3 signalling. A RRC-connection setup (RRC-CS) message 138 is followed by a RRC-CS message on the dedicated control channel DCCH 140, and in turn by an RRC-CS message on the dedicated transport channel (DCH) 142.
An RRC-CS message 178 is then transmit across the air-interface to the UE 150, where it is received and processed. The air-interface (Layer-1) signalling message is converted into a RRC-CS message 180 on the DCH, and in turn an RRC-CS message 182 on the DCCH. Following receipt of the Layer-3 RRC-CS message 184 in the UE, the UE sets up the radio bearer as instructed by the network (within RRC-CS) and initiates a Layer-3 RRC-connection setup complete (RRC-CC) message 185, in turn followed by Layer-2 RRC-CC messages 190, 192. An air-interface RRC-CC message 194 is sent to the network controller 102. The network controller 102 receives the air-interface RRC-CC message 194 and processes this via Layer-2 RRC-CC messages 144, 146 on the DCH and DCCH and Layer-2 RRC-CC message 148 to the RRM 104.
Thus, as illustrated, the known counting procedure used in the UTRAN is complex and requires a large amount of signalling between the UEs and the network controller over the radio air-interface. This results in inefficient radio resource utilisation.
One problem with the aforementioned complex procedures emanates from the fact that the counting procedure used in the UTRAN is designed to determine the transmission mode (between point-to-point (p-t-p) and point-to-multipoint (p-t-m)) of MBMS service based on the number of users in a given cell. Therefore, it is necessary to indicate accurately whether the number of users in the cell is larger than a pre-set threshold value, in order for the network controller to switch to or from p-t-m mode to continually optimise use of resources.
The use of p-t-p transmission mode not only increases the MBMS provision complexity, but also has limited or even non-existent performance gain over p-t-m mode in most of the practical deployment scenarios. This is particularly the case, in orthogonal frequency division multiple access (OFDMA) based Single Frequency Networks (SFN), which are currently being standardised in 3GPP for next generation communication system (Evolved UMTS Terrestrial Radio Access Networks, E-UTRAN).
E-UTRAN employs a single frequency network, where all the base stations are time synchronous. Hence, the same signal can be transmitted from a number of cells in a service area. The service area may contain more than one cell. The transmission of the same signal from time synchronized cells results in over-the air combining of the signal, thus increasing the received signal energy and hence providing better reception at the UE.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of supporting broadcast transmissions over a cellular network would be advantageous. In particular, a system allowing for the provision of broadcast transmissions in an UTRA TDD system to co-exist with the existing UTRA-TDD system would be advantageous.